Forging presses



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INvENTQR KEITH KHQW kAILYPLMHP 'S ATTORN EY Sept. 27, 1966 K. KNOWLES 3,274,819

FORGING PRESSES Filed April 4, 1963 7 Sheets-Sheet 3 7 7 T 26 26 Q Q o u I) 0:0 2 r\ INVENTOR KERR KNOWLES BY A'AM minds ATTORNEY Sept. 27, 1966 K. KNOWLES 3,274,819

FORGING PRESSES Filed April 4, 1963 '7 Sheets-$heet 4 INvEN-roR KEITH Kuowuzs BY Mmk. WM @4 04 ATTORNEY Sept. 27, 1966 K. KNOWLES FORGING PRESSES 7 Sheets-Sheet 5 Filed April 4, 1

INVENTQR KEn-u know LE5 BY MWALQW PM. kmkm ATTORNEY Filed pril 4, 1965 K. KNOWLES FORGING PRESSES '7 Sheets-Sheet 6 INvENToE KEH'H KNOWLES BY LAMA A e \NLMl\\ 6Mfl L on ATTORNEY Sept. 27, 1966 K. KNOWLES FORGING PRESSES '7 Sheets-Sheet 7 Filed April 4. 1963 b 0 & WTM d E w r\ 333 i 11 I lQil I 0 I ll /w ||||x|| m 2 3 3 0 3 g INVENTO'R Mam-b KNOWLES ATTORNEY United States Patent 3,274,819 FORGING PRESSES Keith Knowles, Sheffield, England, assignor to The British Iron and Steel Research Association Filed Apr. 4, 1963, Ser. No. 270,705 Claims priority, application Great Britain, Apr. 4, 1962, 12,840/ 62 4 Claims. (Cl. 72-421) This invention is concerned with improvements in or relating to forging presses and manipulators therefor, and it is amongst the objects of the invention to provide a construction of forging press and of ancillary equipment whereby a group of specified products can be processed, including for example plain cogged bars, rounds, shafts, collars and rolls, and whereby more efficient operation and greater productivity can be achieved through more rapid manipulation and control and co-ordination of the press and the manipulator movements.

Features and advantages of the present invention will appear from the following description of one embodiment given by way of example only, reference being had to the accompanying drawings in which:

FIGURE 1 is an elevation of the forging press;

FIGURE 2 is a plan view of the press;

FIGURE 3 is a section on the line A-A of FIGURE 1;

FIGURE 4 is a section on the line BB of FIGURE 3; FIGURE 5 is a section of the press showing in elevation the manipulator;

FIGURE 6 is a plan view of the tor;

FIGURE 7 is a section of the press on the line BB of FIGURE 6 showing the central workpiece support;

FIGURE 8 is a section of the press on the line A-A of FIGURE 5 showing the tool changing mechanism;

FIGURE 9 is an elevation of the master manipulator;

FIGURE 10 is a plan view of the master manipulator;

FIGURES 11 and 12 are sections on the lines A--A and BB respectively of FIGURE 10';

FIGURE 13 is an end elevation of the .tor; and

FIGURE 14 illustrates schematically a hydraulic control arrangement for the peel drive.

A general arrangement of the press is given in FIG- URES l to 4 which shows the principal components of the press. Two fixed crossheads 1 are separated by two tie bars 2. Two movable crossheads 3 are positionedand supported relative to the fixed crossheads by the tie bars and are moved along them each by a matrix of hydraulic cylinders and pistons 4, or alternatively the more conventional three ram drive. In the first case each matrix is arranged as twelve cylinders 5 and nine pistons 6 on the fixed cr-osshead with the complementary grouping of nine cylinders and twelve pistons on the movable crosshead, the complete assembly forming a five-by-five matrix with each corner cylinder removed. A return cylinder and piston 7 is located at each of the four corner positions. In the second case the three ram configuration may comprise either three pistons of equal size, or be such that the effective ram area of the central piston is equal to four times that of each of the two outer pistons.

To ensure that the two press tools make equal depth of penetration during any one stroke and operate at the same rates, equal volumes of high pressure fluid are automatically supplied to the two sets of drive cylinders to ensure synchronous movement.

Hydraulic oil supplies can be directed through the fixed crossheads with the cylinders in the movable crossheads receiving their supply through special hollow pistons 8, thus eliminating any external pipes to the movable crosshead.

press and manipulamaster manipula- The tie bars may be either round (as shown) or rectangular in cross-section and are positioned so that the vertical distance between them is sufiicient to allow free passage for manipulator jaws holding a workpiece. Each has a solid centre to withstand the complex stresses imposed during forging, and a series of cooling water passages 2a on the outside of this solid centre. The water cooled area of each tie bar extends only between the fixed crossheads.

Provision is made on the movable crossheads for tool changing support slides and locking devices capable of securing and releasing the tools rapidly.

To handle a workpiece during working a manipulator is provided comprising a master 9 (FIGURE 5) and a slave 10. A compound, dual motion drive is provided for longitudinal workpiece positioning .and comprises a manipulator drive for longitudinal positoning of the manipulator which drive is in the form of a. rack and pinion drive as shown at 22, 23, or a hydraulic cylinder drive, say, and a peel drive for longitudinal positioning of the peel relative to the manipulator which drive is in the form of a short stroke hydraulic cylinder 30 to provide rapid movement of a workpiece held by the manipulator peel when forging and also to function as a shock absorber when planishing. The cylinder 30', which has a stroke larger than the maximum bite width is attached to the main frame of the manipulator, its piston rod 300: being connected to the end of the moving peel. The whole manipulator runs on guide rails which restrain it both laterally and vertically.

The master manipulator 9 which grips one end of the workpiece provides all the necessary controlled movements while the slave manipulator .10 which follows the master manipulator peel movements provides support for the other end of the workpieces. To counteract the overturning moment induced by the workpiece, both manipulators are railbound allowing their weight to be reduced by a lighter construction.

The master manipulator includes a peel comprising an outer tube 11 inside which slides the reciprocating peel 12. The jaws for holding the workpiece 13 are located at the free end of the peel. To the other end of the peel is attached the piston rod of the peel actuating cylinder 30 which is secured to the outer tube.

A gear drive 15 rotates the outer tube which is mounted in bearings 16 capable of withstanding the moments induced by the workpiece overhang. The rotational drive is transmitted to the inner tube, or peel by means of splines 17. The housings of the rotational bearings are carried on four trunnions 18 which run in bearings 19 integral with the main manipulator chassis. The trunnions allow the peel assembly to be moved bodily sideways under the action of hydraulic rams 20. The motor 21 for the rotational drive is rigidly mounted between the two bearing housings and this mounting is movable sideways together with the peel assembly.

Two pinion drives 22 are included to enable the racks 23 to be positioned so that scale falling from the ingot falls between the racks and does not impair their operation.

In general, manipulator operation for longitudinal workpiece positioning is as follows: the pinion drive propels the manipulator forward at a constant linear velocity, along the fixed rack. During the press forging stroke, when the workpiece must be stationary, the peel drive is controlled so that the peel is propelled in an opposite direction, at a velocity equal to the sum of the manipulator linear velocity and the velocity caused by ingot extension, with the net results that the manipulator peel remains stationary. Thereafter, during the time commonly referred to as ingot free time, the peel drive 3 is controlled so that the peel is driven rapidly, in the same direction as the manipulator drive, to its starting position relative to the manipulator. This results in a velocity of workpiece longitudinal advance, or bite, equal to the sum of the manipulator and peel drives at the time in which they are both effective in the same direction.

FIGURE 14 illustrates one form of control arrangement for the above double-motion operation. In this figure the peel drive is illustrated schematically as a doubleacting piston-and-cylinder assembly 30 having a piston rod 30a connected to the peel (not shown) at one end and its casing 30d connected to the main frame of the manipulator, and having ports at 30b and 300 for individual hydraulic lines 31 and 32. Hydraulic lines 31 and 32 are connected to individual ports on one side of a 2-position 4-way valve 33 which provides straightthrough fluid flow in the operative position 33a shown, or cross-over fluid flow in its other operative position 33b, relative to two hydraulic fluid lines 34 and 35 connected to two ports on the other side of valve 33. Valve 33 may be of any suitable known form and arranged for switching between its two positions in any convenient manner, mechanically or electrically, say.

Hydraulic lines 34 and 35 are arranged for connection through a 2-position Z-way solenoid operated valve 36. Valve 36 may be operated to the position 36a shown, in which lines 34 and '36 are connected, by energisation of solenoid 360 against the spring pressure of spring 36d. On de-energisation of solenoid 360, the pressure exerted by spring 36d switches valve 36 to become effective in position 36b, whereby the connection between lines 34 and 35 is broken.

Hydraulic lines 34 and 35 are also connected to individual ports on one side of a 2-position 4-way valve 37 normally held by spring 37d in position 37a, as shown, or alternatively held in position 37b by energisation of solenoid 37c. The associated ports on the other side of valve 37 are respectively connected to a hydraulic fluid line 38 for supplying fluid at predetermined pressure on the one hand, and to drain 39 on the other hand.

Assuming that the required peel drive advance between successive press forging strokes or so-called squeezes, is from right to left in FIGURE 14, the valve mechanisms are positioned as shown during squeezes with the peel positioned in a fixed relation to the manipulator at initiation of a squeeze. This position of the peel has been re ferred to above as the starting position but is perhaps better regarded as a datum position which will normally be such that the piston is centrally disposed within its cylinder as indicated in broken outline.

This then indicates the situation on initiation of a squeeze and it will be seen that the piston effectively floats within its cylinder since the supply from line 38 passes through valve 37 to line 34, from line 34 through valve 36 to line 35 and thus through valve 33 to both ends of the cylinder 30. Thus, since the workpiece, the peel, and hence piston 30a are held stationary in space during a squeeze the manipulator moves from right to left relative to its peel in FIGURE 14 at a constant velocity; and conversely, the peel moves in the opposite direction relative to the manipulator at the actual manipulator constant velocity.

On termination of the squeeze, the press tools commence separation and as soon as the tools clear the workpiece solenoid 360 is energised to change valve 36 to position 36b. The result of this is to cut off the supply to cylinder 30 at 30c whereby the piston is driven from right to left by the supply at 30b with the left hand side of cylinder 30 exhausting to drain 39 through lines 32 and 35. The pressure of fluid supply to line 38 will be so chosen that the piston is driven rapidly back to its datum position at least by the time of initiation of the next squeeze. When the piston does reach its datum position solenoid 360 of valve 36 is de-energised so that the valve returns to position 36a whereby the piston floats within cylinder 30 once more, that is to say, the peel then remains stationary relative to the manipulator.

Since, as a result of the above operation, the peel moves from and returns to its datum position between initiation of one squeeze and initiation of the successive squeeze, the actual workpiece longitudinal advance or bite is determined by the constant velocity of the manipulator drive and the squeeze repetition rate or so-called press cycle time. The press cycle time and required bite are substantially constant so that the corresponding constant velocity for the manipulator drive can be readily predetermined.

Considering now control of valve 36 for the above operation: proposals have been made for control of forging press operations by use of electrical signals representing the position of a reciprocable press tool. When the press tool first strikes a workpiece in a pass the tool position indication at that time is employed to set up a corresponding static indication which represents the workpiece initial thickness, and an electrical signal indication representing a predetermined proportion of this initial thickness representation is set up at the time as lower limit for press tool movement in the associated squeeze and subsequent squeezes of the same pass. Thereafter the actual press t-ool position signal is compared, during press tool closure, with the predetermined lower limit signal and the press tool motion is reversed in response to equality between the compared signals.

In the present instance a control signal, generated in response to equality between the above compared signals can be employed to initiate energisation of solenoid 360. This assumes that the response of the peel driving arrangement is such that actual peel drive will be slightly delayed and not effective until the press tools are clear of the workpiece. A sufficient delay for this purpose can be ensured in any event by use of an appropriate electrical signal delay means for the control signal.

In an alternative arrangement the control signal for causing energisation of solenoid 360 may be derived by comparison of the press tool position signal with the initial thickness representation stored at the beginning of a forging pass, the solenoid control signal being generated when these compared representations are equal. However, it is noted that such equality will occur during both tool closure and separation, and the control signal is required only during tool separation. Since in the above-mentioned proposals use is also made of a relay operated to different states in accordance with tool closure or separation, then clearly selection of the correct alternate instants of equality between the compared signals can be readily effected. Again, such an indication of tool separation is available from the increase or decrease of the tool position signal itself. Alternatively, it will be appreciated that electrical circuits are in any event readily available, which circuits are responsive to alternate ones of input signals to generate an output signal.

Turning now to operation of valve 36 back to position 36a when the peel returns to its datum position, a further control signal may be obtained for this purpose by way of a limit switch having a first contact fixed relative to the manipulator and a second contact fixed relative to the peel and movable therewith, these contacts being positioned for engagement to produce the further control signal when the peel is in its datum position. This further control signal is employed to de-energise solenoid 360. In practice it may be desirable that this limit switch is effective to generate the further control signal just before the peel reaches its datum position, so that account is taken of the inertia of the peel drive. This may be achieved by making one of the limit switch contacts of appropriate width in the direction of peel travel, this width being equally spaced on both sides of the theoretically ideal datum position location for that contact.

It will be appreciated in the above operation solenoid 360 is required to be energised in response to a control signal, which will be of pulse form, and retained energised until de-energised in response to a further control signal. This may be effected by application of the control and further control signals as the two inputs of a bi-stable device, or so-called flip-flop, that is an electrical circuit arranged to produce a first continuous output signal when set to one stable state by the control signal, and to produce a second continuous output signal when set to its other stable state by the further control signal. With the form of valve 36 shown in FIGURE 14, position 36a is automatically obtained upon de-energisation of solenoid 360 by the action of spring 36d, and this position will be maintained until solenoid 36c is energised once more. However, the above flip-flop control allows other forms of valve 36 to be employed in which positions 36a and 36b are obtained by energisation of a common solenoid in different senses, or even by use of individual solenoids.

The above description of FIGURE 14 operation and its control has dealt with longitudinal workpiece advance in one direction. A similar operation is appropriate to such advance in the opposite direction simply by making valve 33 effective in position 33b instead of position 33a. The necessary change-over of valve 33 for this purpose will, of course, only be required between successive forging passes.

Mention should also be made of the purpose of valve 37 when in position 37b, and it will be seen that this is effective to lock the peel drive. Such locking is desirable in association with forging operations in which the workpiece is required to remain in a fixed longitudinal position for a succession of squeezes, for example, as in the case with cutting and knifing operations when the workpiece is, at most, only rotated between successive squeezes.

The above compound drive system can be both rapid acting and accurate, since the accuracy depends on the longitudinal positioning of the manipulator relative to the press by the manipulator drive over a long length, and to the final positioning of the workpiece relative to the manipulator by the hydraulic peel drive over a relatively short stroke. Due to the fact that the force exerted by the workpiece on the manipulator is absorbed by the hydraulic cylinder of the peel drive, the peel can stop under momentary overload without damage.

The various motions on the manipulator are operated by a number of hydraulic motors which are supplied from a common hydraulic pump unit which in turn is driven by an electric motor. By this means a single oil reservoir and accumulator assembly can be used. Independent hydraulic motors or actuators and relevant valves are required for each of the following motions:

(a) Gripping jaws.

(b) Rotation drive and detent.

(c) Constant velocity longitudinal manipulator drive.

(d) Transverse positioning of peel.

(e) Peel drive.

The hydraulic pumping unit selected for the manipulator should be capable of accommodating the maximum demand possible, bearing in mind that all motions cannot be operated simultaneously.

Fabrication of the manipulator chassis is such as to assure maximum economy in weight and size to Withstand the complex stresses induced by the forces required to accelerate the peel and to withstand the overturning moments and deadweight loads.

Rigidity of the side members is essential to enable the loads to be transmit-ted from the trunnions to the manipulator track.

The function of the slave manipulator is to support the workpiece, and to position it on the same axis as that of the master manipulator to ensure correct workpiece presentation to the press and also to facilitate forging on the advance and retreat.

In this case a small hydraulic power pack only is required for the three motions which include gripping, transverse positioning and longitudinal drive for movement of the slave manipulator. To keep the weight of this manipulator to a minimum the longitudinal drive equipment could be positioned on the floor rather than on the slave manipulator chassis, as it is only required to enable the manipulator to be positioned to accept the ingot. Thereafter, at least the transverse and longitudinal drives will be disengaged during the forging operation and the slave manipulator will be positioned by the wonkpiece.

In addition to the master and slave manipulators described, a further item of manipulation equipment may be included which is located at the intersection of the railbound manipulator and press forging axis. This takes the form of a central workpiece support and the main functions of this will be:

(a) To assist in the initial loading of the workpiece into the forging press.

(b) To supplement the main manipulator during forging operation.

(c) To enable very short workpieces to be accommodated in the press.

The support comprises a flat-horizontal roller table 25 capable of supporting the full weight of the workpiece. Means are incorporated for driving the rolls, rotating the table about a vertical axis and adjusting the whole support vertically to receive workpieces of varying diameters. Preferably it is completely retractable beneath the floor level when not in use.

There is also provided a tool changing mechanism for each movable crosshead and these include two hinged plates 26 normally forming part of the floor between the fixed crossheads but which can be opened into the vertical position directly beneath the tools. When open, a slide on the underside of each plate interconnects the tool slide on the adjacent movable crosshead and a tool changer mechanism beneath the floor. A vertical ram assembly 27 which forms part of each tool changer can be raised to engage the tool and extracts the tool from the crosshead. The tool is then lowered into a magazine 28 which can be indexed so that the next selected tool is positioned beneath the crosshead for insertion by the vertical ram assembly. A system of spring loaded locking devices retains the tool on the crosshead. It will be seen that the tool changing operation can be effected with the workpiece within the press structure.

The system of control for the integrated forging unit will require initially, a complete forging schedule .to be determined and fed into the control unit. When initiated, the forging unit will perform the calculated pass sequences until the whole programme has been completed. Information such as workpiece size and length, and datum positions of both manipulators can be set and stored in digital form so that together with the original forging schedule it can be fed back to the control system during the forging operation. Manual control of all motions should be available to enable the initial setting up and the determination of datum positions of the ingot to be made.

With two manipulators, each holding one end of the ingot, complete reference to the length and position of the ingot relative to the press tool centre line is continuously available. When a set-down or knifing pass is to be made, fine positioning is achieved by the location of the manipulator by the manipulator drive with the peel locked in its datum position as described above. The positional accuracy of the workpiece in its longitudinal axis is not critical when cogging; consequently for this operation no complicated control equipment is necessary to control the exact position of the workpiece other than for its end limits.

The forging press described and shown presents various advantages.

Horizontal features (a) Low headroom of ft. compared with the 38 ft. required by a Push-down press and the shallow foundations ft. compared with the 36 ft. necessary for a similar capacity Draw-down press. These dimensions relate to a press capacity of 2,500 tons for working ingots up to 24" square or 30" octagon and weighing 8 tons.

(b) The press will be more accessible for maintenance work much of which should be accomplished from floor level. The crosshead can be readily withdrawn from the backing plate along the tie bars to reveal the pistons and cylinders for easy maintenance work and hydraulic seal renewals.

(c) Although at first it would seem that more floor area is necessary for a horizontal press, the disadvantage of increasing deadweight load area is offset by a reduction in working area; the area at present required for marking off and tool handling is no longer necessary.

(d) Tool changing becomes simplified with a horizontal press; all tools can be extracted and replaced from a tool magazine located directly beneath the axis of operation of the tools and this operation can be speedly effected whilst the ingot remains within the press on its fixed forging axis.

(e) Stability is increased because the centre of gravity of the press is beneath the tool axis and the press covers a relatively large bearing surface.

(f) By introducing a side squeeze, the underside of the ingot on the plane of squeezing is free and a new approach to ingot support and manipulation is possible.

Double-acting feature (a) The centre line axis of the ingot remains constant during the entire forging operation; this simplifies the manipulator design and saves valuable forging time by eliminating the vertical motion.

(b) The movable crosshead speed and hence the inertia forces will be half that of a single-acting press.

(c) The piston rods on a double-acting press will only be half as long as those on a single-acting press for the same depth of penetration stroke. Consequently the stresses due to bending caused by the piston overhang will be substantially reduced. The reduced length of piston rod will also affect its deflection and result in a reduction of hydraulic seal wear.

Two-column feature A two column press where the tie-bars are positioned on an axis oflset from the vertical allows the manipulator a greater degree of accessibility into the working area of the press tools. This should enable very short ingots to be forged and also enable all ingots to be forged to their full extremity in length. The use of two columns permits a saving in size of both fixed and movable crossheads with an attractive economy in the overall size and weight of the press.

Multi-cylinder high pressure hydraulic drive The use of high pressure hydraulics results in smaller piston diameters for corresponding thrust values. By using a number of small cylinders a more even load distribution can be obtained behind the press tool, and any combination of cylinders symmetrical about the central axis of the tools can be used to provide a variety of press capacity ratings. Furthermore it is quite feasible that if a cylinder seal leaked during -a forging operation, the hydraulic supply to that cylinder could be curtailed for the duration of the production run with only a marginal reduction in performance of the press.

Manipulation (1) Support is provided for the ingot at both ends, so that it will always be presented to the tools on a fixed horizontal axis.

(2) Rapid and accurate positioning of the ingot is obtained in the longitudinal plane and about its longitudinal ax1s.

(3) The peel can be stopped under momentary overloads without damage.

(4) The ingot can be moved transversely during initial setting up.

(5) By making the manipulator railbound a lighter manipulator structure is achieved which obviates the need to make a massive structure to counter the overturning moment produced by the axially extending ingot. Furthermore a fixed centre line is achieved for the ingot travel during forging.

I claim:

1. A workpiece manipulator for a forging press, comprising a carriage (9, 10), carriage drive means (22-23) for continuously driving said carriage during a forging pass, a peel (12) mounted on said carriage, and peel drive means (30-39) for driving said peel longitudinally relative to said carriage, said peel drive means comprising a double-acting piston-and-cylinder assembly (30) one element (30a) of which is connected to said peel and V the other of which is connected to said carriage and comprising valve means (33, 36, 37) operable in a first state in which fluid supplied to both ends of the cylinder for equal and opposite drive forces on the piston whereby said piston eflectively floats within said cylinder to permit, during a squeeze, relative movement of said peel and said carriage from a datum position, and operable in a second state in which fluid is supplied to one end of the cylinder and is exhausted from the other end thereof to return, between squeezes, said peel and said carriage to said datum position.

2. A manipulator according to claim 1 comprising two fluid lines (31, 32) respectively connected to the two ends of said cylinder, a common fluid supply line (38, 34) connected to a first one of said two fluid lines, a fluid drain line (35, 39) connected to the second one of said two fluid lines, and wherein said valve means includes a first valve mechanism (36) operable to open and closed states and connected between said common fluid supply line and said second fluid line.

3. A manipulator according to claim 2 wherein said valve means further comprises a second valve mechanism (33) connected in said two fluid lines, and operable in first and second states for straight-through and crossover connections of said two lines, respectively, relative to said common fluid supply line and said fluid drain line.

4. A manipulator according to claim 3 wherein said valve means additionally comprises a third valve mechanism (37) connected in said common fluid supply line and said fluid drain line, and operable to open and closed states.

References Cited by the Examiner UNITED STATES PATENTS 1,810,698 6/1931 Diescher 72-421 2,114,302 4/ 1938 Harter 72-402 2,519,900 8/1950 Geiger 91-207 2,619,695 12/1952 Young 91-207 2,741,374 4/1956 Morgan 214-26 2,769,298 11/1956 Jones 56-328 2,808,885 10/1957 Tomka 200-264 3,073,190 1/1963 Appel 72-241 3,209,578 10/1965 Muller 72-421 CHARLES W. LANHAM, Primary Examiner. 

1. A WORKPIECE MANIPULATOR FOR A FORGING PRESS, COMPRISING A CARRIAGE(9,10), CARRIAGE DRIVE MEANS (22-23) FOR CONTINUOUSLY DRIVING SAID CARRIAGE DURING A FORGING PASS, A PEEL (12) MOUNTED ON SAID CARRIAGE, AND PEEL DRIVE MEANS (30-39) FOR DRIVING SAID PEEL LONGITUDINALLY RELATIVE TO SAID CARRIAGE, SAID PEEL DRIVE MEANS COMPRISING A DOUBLE-ACTING PISTON-AND-CYLINDER ASSEMBLY (30) ONE ELEMENT (30A) OF WHICH IS CONNECTED TO SAID PEEL AND THE OTHER OF WHICH IS CONNECTED TO SAID CARRIAGE AND COMPRISING VALVE MEANS (33,36,37) OPERABLE IN A FIRST STATE IN WHICH FLUID SUPPLIED TO BOTH ENDS OF THE CYLINDER FOR EQUAL AND OPPOSITE DRIVE FORCES ON THE PISTON WHEREBY SAID PISTON EFFECTIVELY FLOATS WITHIN SAID CYLINDER TO PERMIT, DURING A SQUEEZE, RELATIVE MOVEMENT OF SAID PEEL AND SAID CARRIAGE FROM A DATUM POSITION, AND OPERABLE IN A SECOND STATE IN WHICH FLUID IS SUPPLIED TO ONE END OF THE CLINDER AND IS EXHAUSTED FROM THE OTHER END THEREOF TO RETURN, BETWEEN SQUEEZES, SAID PEEL AND SAID CARRIAGE TO SAID DATUM POSITION. 